1. Field of the Invention (Technical Field)
The present invention relates to high strength couplings and methods for making the same, and primarily couplings as for use in connecting casing pipe, tubing and sucker rods in oil and gas wells.
2. Background Art
The various components of oil well casings, tubing and sucker rod strings must often operate under severe corrosive and loading conditions. Presently, couplings used for connecting these components, which are made of carbon or alloy steel, very frequently fail because of corrosion attack, wear or fatigue fracture. In order to minimize failure where corrosion is encountered, a corrosion inhibitor is frequently injected into the well in an effort to provide corrosion protection. But in many wells high operating temperatures render protective inhibitors useless and ineffective.
Another method to overcome corrosion attack is to apply a corrosion and wear resistant metallic coating on the outer surface of the coupling. These coatings are usually applied with thermospraying techniques called metallizing. This process, however, also softens the steel, thereby reducing the strength level and making the coupling more susceptible to fatigue fracture. In order to increase the strength, and in particular, the fatigue resistance in the threaded portion of these couplings, the threads are roll-formed after metallizing is completed, to induce compressive stresses in the thread roots, thus increasing the fatigue life of the coupling. If the coupling is roll-threaded before the metallizing process, the compressive stresses are tempered back during the subsequent metallizing process, reducing the fatigue resistance in the threaded roots where fatigue fracture very often initiates. Therefore, in order to permanently induce compressive stresses in the thread roots, roll-threading is employed only after metallizing or heat treating is completed. Although roll-threading helps greatly in increasing the fatigue life in the coupling, it is a costly process, making the couplings more expensive because of the additional power required to roll-thread compared to that required by machine cutting the threads. In couplings used presently for sucker rod strings, however, the roll-threading process is relatively inexpensive, because the coupling hardness level is still soft enough, about 20 Rockwall xe2x80x9cCxe2x80x9d (HRC), to roll-thread with a reasonable amount of power and tool life. If, however, the coupling strength is increased by heat treating, say above about 23 HRC, then the roll-threading process becomes very costly because of the greater turning power and number of roll-forming tools required for roll-threading. This is one reason higher strength couplings have not been developed beyond a strength level of about 23 HRC.
There are a number of prior art methods that address aspects of this problem. Thus U.S. Pat. No. 5,196,075, to Jansen et al., teaches a method of producing a hard corrosion resistant metallic coating on fasteners and screws whereby the nickel or cobalt coating is heated above 850xc2x0 C. so as to diffuse into the steel matrix. However, high core or fatigue strength is not maintained in this invention. U.S. Pat. No. 4,905,760 to Gray teaches a method of applying a plastic coating on the sucker rod coupling outer surface in order to prevent corrosion of the steel underneath it. Again, neither high core hardness nor fatigue strength is thereby maintained. U.S. Pat. No. 4,871,020, to Rivas et al., teaches a method of incorporating roller bearings on the outside of very elongated sucker rod couplings in order to minimize friction and coupling wear. This does not, however, address the inherent core or fatigue strength of the coupling. In other approaches, such as that disclosed in U.S. Pat. No. 4,757,861, to Klyne, a non-metallic sleeve centralizer is connected between sucker rod couplings.
A series of patents to Hermanson et al., including U.S. Pat. Nos. 5,334,268, 5,405,457, and 5,405,461, (hereafter xe2x80x9cHermanson et al.xe2x80x9d) teach a method of making a high strength coupling wherein the coupling is heat treated to a hardness between 32 and 36 HRC. In this step, the coupling is heated and quenched in a salt bath maintained at a temperature below the martensite start (MS) temperature. This heat treatment, commonly called xe2x80x9cMartemperingxe2x80x9d or xe2x80x9cMarquenchingxe2x80x9d, results in transformation of the steel microstructure to essentially martensite, a hard, brittle, body-centered tetragonal structure. The coupling threads are partially completed by machine cutting and subsequently finished by roll-threading. The partial roll-threading induces very shallow compression stresses, about 0.003 inch deep, into the thread root.
In can thus be appreciated that there is a need for a coupling and method for making the same which possesses exceptional performance characteristics in corrosive environments together with very high resistance to fatigue fracture at a more reduced cost than is presently available.
The invention provides a threaded coupling, which is a cylindrical steel core having an inner surface case with a plurality of threads thereon and an outer surface, the outer surface including a metallic coating, where the interior of the steel core wall has a microstructure consisting primarily of bainite, and the inner surface case with a plurality of threads has a microstructure consisting primarily of a mixture of martensite and bainite. In this threaded coupling, the interior of the core well may have a Rockwell hardness of at least about 25 HRC and the inner surface case with a plurality of threads may have a Rockwell hardness of at least about 30 HRC. In one embodiment, the interior of the core well has a Rockwell hardness of at least about 30 HRC and the inner surface case with a plurality of threads has a Rockwell hardness of at least about 33 HRC.
In the threaded coupling, the steel includes carbon and iron, and optionally manganese, nickel, chromium, molybdenum, silicon or boron. The metallic coating of the coupling can include a corrosion and wear resistant metallic coating. This corrosion and wear resistant metallic coating can be made from carbon, iron, nickel, chromium, molybdenum, cobalt, tungsten, silicon, boron, aluminum or a combination thereof.
The invention also provides a method of making a corrosion resistant threaded steel coupling with high fatigue strength, which method includes the steps of providing a cylindrical steel coupling of defined wall thickness having an inner surface and an outer surface; coating the outer surface with a corrosion and wear resistant metallic coating; threading the inner surface to finished dimension with a plurality of threads; carburizing the cylindrical steel coupling, whereby the carbon content of the threaded inner surface is increased; and heat treating the cylindrical steel coupling by austempering, whereby the hardness of the threaded inner surface is increased. In this method, after heat treating the interior of the wall thickness of the cylindrical steel coupling has a microstructure consisting primarily of bainite, and the threaded inner surface has a microstructure consisting primarily of a mixture of martensite and bainite.
In one embodiment of this method, the interior of the wall thickness of the cylindrical steel coupling has a Rockwell hardness of at least about 25 HRC and the threaded inner surface has a Rockwell hardness of at least about 30 HRC, and preferably the interior of the wall thickness of the cylindrical steel coupling has a Rockwell hardness of at least about 30 HRC and the threaded inner surface has a Rockwell hardness of at least about 33 HRC.
The steel of the steel coupling includes carbon and iron, and optionally one or more of manganese, nickel, chromium, molybdenum, silicon or boron. The corrosion resistant metallic coating can be made of carbon, iron, nickel, chromium, molybdenum, cobalt, tungsten, silicon, boron, aluminum, or a combination thereof. The method of coating can include thermospraying, fusing, diffusing, electroplating, vapor deposition or welding. The method of carburizing can include heating above about 1400xc2x0 F. for at least about thirty minutes in an atmosphere with a carbon potential of at least about 0.35%, and preferably with a carbon potential of at least about 0.60%.
In this method, heat treating by austempering includes heating to an austenitic transformation temperature for at least about thirty minutes, rapidly cooling into a salt bath maintained at a temperature above the martensite start temperature but below the pearlite formation temperature, maintaining in the salt bath for at least about two minutes, and cooling to room temperature. In another embodiment, heat treating by austempering includes maintaining in the salt bath for at least about four minutes. The treating by austempering process can further include subsequent reheating to a temperature of at least about 200xc2x0 F. for at least about five minutes to temper the microstructure following cooling to room temperature.
Accordingly, a primary object of the present invention is to provide a process by which couplings made of carbon or alloy steels with optimized alloying compositions can be carburized and heat treated by austempering to increase the overall strength of the core while simultaneously or subsequently increasing for fatigue strength of the threads.
Another object of the present invention is to provide a process for making couplings from carbon or alloy steels that can withstand high load stresses and corrosive fluids in a superior manner.
A further object of this invention is to provide a process for making couplings, including downhole couplings such as sucker rod couplings, wherein the coupling is coated with one or more wear and corrosion resistant materials, threaded by machining, carburized and austempered, to form a corrosion resistant high strength coupling.
Another object of the present invention is to provide a method of manufacturing a coupling to provide enhanced corrosion protection, by metallizing the outer surface with corrosion resistant alloys, thereby forming a coupling which resists abrasive wear and corrosive fluids, which coupling is carburized and austempered subsequent to metallization.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawing, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.